Synonyms for Penstock (same or very similar meaning)
WordNet sense 1 (regulator consisting of a valve or gate that controls the rate of water flow through a sluice):
floodgate, sluicegate, head gate, sluice valveWordNet sense 2 (conduit that carries a rapid flow of water controlled by a sluicegate):
sluice, sluicewayFrom the ODE community, based on WordNetadd/edit